1. Field of the Invention
The present invention relates to a high-frequency signal line, and more particularly to a high-frequency signal line preferably for use in high-frequency signal transmission.
2. Description of the Related Art
As a conventional high-frequency signal line, for example, a signal line disclosed in WO 2011/018934 is known. FIG. 9 is an exploded view of the signal line 500 disclosed in WO 2011/018934.
The signal line 500 illustrated in FIG. 9 includes a body 512, ground conductors 530a, 530b, 534, and a signal line 532. The body 512 is made by laminating insulating sheets 522a through 522d in this order.
The signal line 532 is provided on the insulating sheet 522c. The ground conductors 530a and 530b are provided on the insulating sheet 522b. The ground conductors 530a and 530b are located on opposite sides of a slit S. The slit S, when viewed from the direction of lamination, is located over the signal line 532. Accordingly, the ground conductors 530a and 530b are not opposed to the signal line 532.
The ground conductor 534 is provided on the insulating sheet 522d and is opposed to the signal line 532 with the insulating sheet 522c located therebetween.
In the signal line 500 having the structure described above, the ground conductors 530a and 530b are not opposed to the signal line 532, and there is little capacitance between each of the ground conductors 530a and 530b and the signal line 532. Therefore, a reduction in the distance in the direction of lamination between each of the ground conductors 530a and 530b and the signal line 532 is unlikely to cause too large an increase in the capacitance between each of the ground conductors 530a and 530b and the signal line 532, and is unlikely to result in a shift of the characteristic impedance of the signal line 532 from a desired value. Accordingly, with the structure of the signal line 500, it is possible to make the body 512 thinner.
In the signal line 500 disclosed in WO 2011/018934, however, relatively low-frequency noise is likely to occur as described below. In the following, both end portions of the signal line 500 are denoted by reference symbols 540a and 540b, and the portion of the signal line 500 between the end portions 540a and 540b is referred to as a line portion 542.
As is apparent from FIG. 9, the line portion 542 of the signal line 500 has a homogeneous cross-section structure. Accordingly, the characteristic impedance of the signal line 532 in the line portion 542 is constant. The end portions 540a and 540b are, for example, inserted in sockets of a circuit board. In this regard, both end portions of the signal line 532 are connected to terminals in the sockets, and parasitic impedance occurs in these connection portions. Moreover, the portions of the signal line 532 in the both end portions 540a and 540b are opposed to the conductors in the sockets, and parasitic capacitance is created between each of the parts of the signal line 532 in the end portions 540a and 540b and each of the conductors in the sockets. As a result, the characteristic impedance of the signal line 532 at the end portions 540a and 540b becomes different from the characteristic impedance of the signal line 532 at the line portion 542.
Then, when the characteristic impedance of the signal line 532 at the end portions 540a and 540b is different from the characteristic impedance of the signal line 532 at the line portion 542, reflection of a high-frequency signal occurs at the end portions 540a and 540b. This causes a low-frequency standing wave with a half wavelength equal or substantially equal to the distance between the end portions 540a and 540b. Consequently, low-frequency noise is radiated from the signal line 500.